(1) Field of the Invention
The present invention relates generally to a device and method for measuring the distance to a target, and in particular to the processing of low power signals comprised of a plurality of energy pulses to determine the distance to a target with a high degree of accuracy, particularly when the distance to the target changes over a period of time.
(2) Description of the Prior Art
Various devices and related methods are known in the prior art for determining the distance to a target. Generally, these devices include a transmitter to send a signal to the target, a receiver to capture the echo or reflected signal from the target, and a processor for calculating the distance from the transmitter to the target based upon the time between the signal's departure and return is well known in the prior art. Various energy sources have been used to generate the signal, including sound, light, and radio waves. Most of these prior art devices require a large power source and elaborate equipment in order to be operable.
Recently, there has been considerable interest in radar systems based upon the use of low and micropower energy sources that can be operated from batteries. For example, U.S. Pat. Nos. 5,345,471, 5,465,094 and 5,523,760 to McEwan, incorporated herein in their entirety, describe a system using an ultra-wideband signal. While, other systems using ultra-wideband spectrum signals have been described in the prior art, accurate measurement of the time of return of the signal echo, due to the low power and high frequency of the signal, has been inadequate for practical uses.
In U.S. Pat. Nos. 5,661,490, 5,517,198 and 5,610,611, incorporated herein by reference, McEwan addresses this problem by time elongating the return signal, so that the signal data is effectively slowed down to allow for processing. Specifically, McEwan converts data relating to a plurality of identical return pulses, into the representation of a signal pulse identical in shape to a single pulse, but elongated over the time of the combined pulses. Therefore, with the ability to calculate with accuracy the return time of a pulse, and knowing the time of departure of the pulse, it is possible to calculate, with some degree of accuracy under controlled conditions, the distance from the signal transmitter to the target.
However, while the McEwan signal processing method is functional under controlled or laboratory conditions, it has been found that the method without modification is not commercially viable under outdoor environmental conditions, due to uncontrollable factors such as those encountered in measuring river and lake levels. Specifically, when using low power pulses of the frequency contemplated by McEwan, the return signal is usually mixed within electrical interference or "noise" that is also captured by the receiver, preventing accurate identification of the signal or an accurate determination of the time of return. Since the return signal is of such low intensity with this technology, prior art techniques for separating the signal from the interference are not useful.
Other factors not relevant to laboratory conditions, in particular temperature changes, also prevent measurements with the high degree of accuracy required for many potential applications of the McEwan method. Under laboratory conditions, the time when the signal transmission is initiated can be used as the base or starting point for time measurement, since the time increment between signal initiation and actual transmission under the same conditions is not a variable. However, changing temperature conditions may be experienced when using the system in uncontrolled environments.
If so, the time between signal initiation and actual signal transmission with vary due to electronic draft and component variation. While this variation may be only on the order of about 0.05 nanoseconds, the effect on measurement accuracy can be significant. For example, a temperature change of 10.degree. C. can change the time of signal transmission by 0.25 nanoseconds. When attempting to measure a target located at a distance of 3 meters, this temperature change can result in an error of as much as 3 centimeters.
Many potential applications of radar are in outdoor environments. If so, the device may be damaged, or its function impaired, by rain, snow, insects, and other foreign matter present in outdoor environments. Therefore, the device must be designed in a way that prevents entry into the apparatus of foreign matter. Ideally, the entire device would be sealed within a container with no openings or apertures. Normally, however apertures are required for egress of the transmitted signal and ingress of the return signal. These openings are of primary concern in designing a device that is impervious to external conditions and foreign matter.
Numerous applications exist for radar devices using low power signal sources, provided that the preceding factors limiting use of this technology in external and other uncontrolled environments can be overcome.